Aug 1, 2008 12:00 PM, By Harold Kinley

Their design and construction require tradeoffs between insertion loss and selectivity

Quarter-wave resonant cavity filters are used in many RF applications in land mobile radio. For instance, they are used in duplexers for repeaters, in transmitter combiners, and as filters for specific interference problems. It is important to understand how the various specifications impact the practical use of these cavities. Generally, when filters are required, two criteria must be met — maximum rejection or attenuation at one frequency (or frequency band) and minimum insertion loss at another frequency (or frequency band).

For this article, the term insertion loss is defined as the unintentional loss at the desired pass frequency or band, while rejection is the desired loss at the undesired frequency or band. The goal is to minimize the insertion loss at the desired frequency and maximize the attenuation or rejection at the undesired frequency. This article focuses on the practical aspects of cavity filters rather than the theoretical.

Figure 1 shows a bandpass cavity using dual-loop coupling. The first thing you will notice is the quarter-wave conductor extending down from the top into the cavity. Turning the knob on top will change the length of this conductor (hence, the resonant frequency). The threaded rod can increase or decrease the length of the conductor by the extender section that is connected to the threaded rod. The extender section makes contact with the main conductor section through contact fingers located at the bottom of the main section.

The top end is connected to ground (cavity case) and the opposite end of the conductor is open. The section near the grounded end is low impedance (high current) while the opposite end is high impedance (high voltage). The coupling loops are located near the high current, low impedance (magnetic) end of the conductor. The walls of the cavity are usually silver plated to increase the conductivity and hence the “Q” of the cavity. The cavity Q is also largely dependent upon the volume of the cavity. Because operating frequency range determines the length of the cavity, only by increasing the diameter of the cavity can the volume be increased. Hence, higher cavity Q requires a larger diameter cavity.

Since the cavity is symmetrical, either coupling loop can serve as the input or the output. Photo 1 shows a coupling loop removed from a cavity. Notice that the loop is made of wide copper strips to provide more conducting surface. This minimizes the skineffect at higher radio frequencies. The amount of coupling between the loop and the cavity affects both the insertion loss at the desired pass frequency and the selectivity. An ideal cavity filter would introduce zero insertion loss at the desired pass frequency and infinite loss at frequencies outside the desired passband. However, in practical cavity design and construction, tradeoffs have to be made between insertion loss and selectivity.

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